A non-aqueous electrolyte secondary battery has characteristics of high working voltage and high energy density compared to a conventional nickel-cadmium secondary battery or the like, and it has been broadly used as a power supply of electronic equipment. A lithium transition metal complex oxide typified by lithium cobaltate, lithium nickelate, lithium manganate or the like has been used as a positive electrode material of the non-aqueous electrolyte secondary battery.
Among these, lithium manganate has advantages such that a raw material is easily obtained with a low cost and that it has lower impact to the environment because a large amount of manganese exists as a resource which is a constituent element of lithium manganate. Because of this, a non-aqueous electrolyte secondary battery using lithium manganate has been conventionally used in mobile electronic equipment typified by mobile phones, laptops, or digital cameras.
Because of enhancement of the functions of the mobile electronic equipment such that various functions are added, its use at high temperature or low temperature or the like, the required characteristics of a non-aqueous electrolyte secondary battery used in the mobile electronic equipment has been demanded more and more in recent years. Further, a non-aqueous electrolyte secondary battery is expected to be used as a power supply of a battery for an electric car or the like, and a battery has been demanded which is capable of high-output and high-speed discharge to be able to follow the quick-start and quick-acceleration of a car.
Because of that, attempts have been carried out to improve the smooth insertion and release function of lithium ions by making the average particle size of the positive electrode active material particles such as lithium manganate particles small. For example, a process is disclosed in Patent Document 1 described below for producing lithium manganate having an average preliminary particle size of 0.01 to 0.2 μm and an average secondary particle size of 0.2 to 100 μm by mixing manganese oxide having an average preliminary particle size of 0.01 to 0.2 μm with a lithium compound and the like to be fired, and then pulverizing the mixture.
However, it is difficult to obtain a diffusion space that is enough for lithium ions to be smoothly inserted and released only by making the average particle size of the positive electrode active material particles small or by controlling the average particle size of aggregate particles as in the above-described production process. Further, when producing a positive electrode using the positive electrode active material particles, there is a problem that it is difficult to secure a diffusion space of lithium ions with stability due to mixing of a binder and the like or due to making the particles into a paste.
Then, there is an attempt for actively forming a space by making the positive electrode active material particles porous besides the space generated in a gap between the positive electrode active material particles for the purpose of expanding the diffusion space of lithium ions.
For example, a process has been proposed in Patent Document 2 described below for producing positive electrode active material particles in which porous particles are formed by producing a mixture containing preliminary particles of a lithium-containing complex oxide and pore-forming particles and then by removing a constituent material of the pore-forming particles contained in the mixture. On this occasion, a process is disclosed for removing a part of the constituent material by using resin particles such as polystyrene particles as the pore-forming particles, heating the mixture to 300 to 600° C., and thermally decomposing the resin particles.
However, in the production process described in Patent Document 2, it has been found that the structure of the mixture after heating is not stabilized, and the pore-forming property is not sufficient, and the charging and discharging characteristics can not be improved sufficiently when constituting a positive electrode composition for a battery by further pulverizing the mixture. In the production process described in Patent Document 2, because the positive electrode active material particles are bound to each other by thermally decomposing the resin particles that are the pore-forming particles and then leaving apart of the particles, it has been found that the resin and the like are easily left also on the surface of the positive electrode active material particles and that the remained component can easily become a hindrance to insertion and release of lithium ions on the surface of the positive electrode active material particles.
On the other hand, a process is disclosed in Patent Document 3 described below for granulation by a spray drying method using a lithium salt such as lithium carbonate as an open pore-forming agent. However, a specific process using resin particles is not described.